In systems that employ a reflective optical device for the processing of light there is typically a path through which the illumination light beam travels and a path through which the reflective light beam travels. These respective paths are referred to as the illumination path and the reflection path. When lenses or other optical elements are positioned to receive the reflective light beam, these elements must be arranged relative to the illumination path so that they do not interfere with the incoming illumination beam. For example, in an optical projection system, a Total Internal Reflection (“TIR”) prism may be used to separate the illumination path from the reflection path. The reflection path would be referred to in this embodiment as a “projection path.” The TIR prism may formed of two prism elements adjacent to each other, with a small air gap between them.
In one embodiment, the illumination beam is reflected at the air gap of the TIR prism using TIR reflection and is thereby directed towards the reflective optical device, which may be, for example, a Spatial Light Modulator (“SLM”), which modulates the illumination light and provides a modulated, reflected light beam that may carry an image, data, or other signal modulated on the light beam. This modulated, reflected beam is directed at a high incident angle such that it passes through the TIR surface of the TIR prism and through the air gap therein. If the light beam hits the TIR surface at less than the critical angle, it is reflected. This principle is used to reflect away the unwanted light components, such as from off-state pixels of the SLM or from other structures and surfaces of the SLM or other optical components in the system that are reflecting the light at angles outside of the desired projection angle. The light that passes straight through the TIR prism after reflection by the SLM remains on the reflection or projection path, and is typically passed through a lens or through other optical elements. The lens or other optical elements may, for example, project the light beams onto a display surface or direct them to other lenses or other optical elements.
One projection system is the Digital Light Processing (“DLP”) projection system manufactured by Texas Instruments for use by display and projector manufacturers. The optical processing elements in the DLP projection systems are referred to as Digital Micromirror Devices (“DMD”), and they comprise hundreds or thousands of individual reflective pixel elements which, depending on electrostatic forces placed on the individual pixel elements, reflect the illumination light on to the projection path or to a separate “off-state” path. The aforementioned TIR reflection surface is used to reflect off these off-state light reflections, as well as to reflect away light beams originating from extraneous surfaces such as the DMD supporting structures, glass windows, prisms or flat-state pixels, which are pixels not deflected at the ideal reflection angles. Once the desired light beams have been directed to the projection path, they are typically received by a lens, which focuses the beams either on to a projector or subsequent optical elements in the projection path.